The present disclosure is directed generally to digital optical transmission systems, and more specifically to high-speed digital optical transmitters employing enhanced chirp control techniques to improve data transmission performance.
Optical transmission systems include optical transmission media, such as an optical fiber, for propagating information between an optical transmitter and an optical receiver. An optical transmitter for an optical fiber transmission system includes an optical source, such as a semiconductor laser, for generating a Continuous Wave (CW) light beam, and an optical modulator for modulating the CW light beam with the information to be transmitted. The optical receiver detects the transmitted optical signal and processes the optical signal into an electronic waveform that contains the transmitted information.
Chirp refers to the spread of the optical frequency of a signal. In optics, chirp may be caused due to the dispersion of the signal as it propagates through the optical fiber. Digital optical transmission systems are known that employ chirp control techniques to improve data transmission performance. For example, chirp may be introduced in the CW light beam to offset dispersion caused by the properties of the optical fibers.
Because the electrical data signal input to the modulator typically has a Non-Return-to-Zero (NRZ) data format, the optical signal generated by the data modulator typically has an NRZ modulation format. Modulators are also known to employ a Return-to-Zero (RZ) modulation format in digital optical transmission systems. Like the NRZ data transmission system, the conventional optical transmission system employing the RZ modulation format includes a laser for generating a CW light beam, and a data modulator for modulating the CW light beam in response to an electrical NRZ data signal. In addition, the conventional RZ data transmission system includes an RZ pulse modulator for carving RZ pulses from the modulated optical signal carrying the NRZ data.
Conventional NRZ or RZ optical transmission systems operating at bit rates of about 10 Gbits/s typically deploy either negative chirp (i.e., decreased optical frequency at leading edges of the modulated optical signal and increased optical frequency at trailing edges of the modulated signal) or no chirp (i.e., essentially no change in the optical frequency of the modulated optical signal) at the optical transmitter when transmitting data over optical fiber having positive dispersion characteristics. This is to achieve what is commonly known as the “maximum dispersion distance,” which is the fiber distance beyond which neighboring data bits start to overlap and interfere. For example, the maximum dispersion distance for the conventional 10 Gbits/s optical transmitter is approximately 60 km of Standard Single-Mode Fiber (SSMF).
Another fiber distance that impacts optical transmission performance is the “effective non-linear fiber distance,” which is the fiber distance over which the optical signal power is high enough to introduce impairment from fiber non-linearity. For example, the effective non-linear fiber distance for the conventional 10 Gbits/s optical transmitter is approximately 20 km of SSMF. In general, for optical transmitters operating at bit rates up to about 10 Gbits/s, the maximum dispersion distance is longer than the effective non-linear fiber distance. Because of the interplay between dispersion and fiber non-linearity in the transmission of optical data, conventional NRZ or RZ optical transmission systems operating at bit rates of about 10 Gbits/s typically employ both chirp control (e.g., negative chirp or no chirp) and dispersion mapping (e.g., placing dispersion compensating fiber at certain positions in the transmission link) techniques to optimize the overall data transmission performance.
For the foregoing reasons, it may be desirable to introduce chirp into a modulated CW light beam produced by commercially available components, such as a dual-drive Mach-Zehnder transmitter (Lithium Niobate or Indium Phosphide) that are designed for zero chirp, or to compensate for chirp introduced by the modulator.